Rechargable batteries are used extensively to power portable electronic devices such as portable or cellular phones and laptop computers. When using a device powered by a rechargeable battery, it is desirable to know how much usable charge remains in the battery. As the capacity of a rechargeable battery diminishes with each recharging cycle, accurately predicting the remaining charge requires knowing how many times the battery has been recharged. Nickel cadmium (NiCd), and to a lesser extent nickel metal hydride (NiMH), rechargeable batteries also suffer from “voltage depression”, an affect in which the battery voltage drops unusually quickly if they are recharged before being fully discharged. For optimum recharging of NiCd and NiMH batteries, it is, therefore, desirable to know the life-time charge history of the battery, particularly the state of discharge it reached during use.
To meet these battery monitoring needs, a number of smart, rechargeable batteries have become available that incorporate memory and circuitry for calculating, storing and reporting a charge status and history of the battery. Such smart, rechargeable batteries are described in, for instance, U.S. Pat. No. 5,600,230 to Dunstan, entitled “Smart Battery Providing Progammable Remaining Capacity and Run-time Alarms Based on Battery Specific Characteristics”, the contents of which are hereby incorporated by reference.
Certain Land Mobile Radio (LMR) portable radio applications, however, have battery monitoring requirements that go beyond what is provided by current smart rechargeable battery technology. For instance, emergency and rescue services require that any status storing and reporting technology added to rechargeable batteries used to power their LMR portable radios must have no detrimental effect on the battery. This means that the status storing technology must provide data retention for as long as five years, without drawing any power from the rechargeable battery during that time, under a wide range of environmental conditions. In addition, the status storage technology must allow for sudden and rapid removal from charging devices and equipment without loss of status data or any other significant information.
Attempting to meet these stringent requirements results in conflicting technology needs. For instance, the issue of avoiding data loss (or having unacceptably out of date data) when a battery pack is removed from a device may be avoided by reporting battery status to the memory on a regular, quasi-continuous, schedule (about once every 30 s). In a typical 9 hour battery recharge this would result in about 1000 memory erase/write cycles. The problem with this solution is that, to meet the low cost, long term storage requirements, the most suitable memory is an Electrically Erasable Programmable Read Only Memory (EEPROM) device, but EEPROMs have a finite number of erase/write cycles (typically 50,000) before they fail. (The erase function degrades an oxide barrier on the silicon and eventually leads to failure of the EEPROM). This finite number of erase/writes of the EEPROM would limit the total number of battery charge-discharge cycles to an unacceptably low 50 cycles, if the battery status was updated on a quasi-continuous schedule.
To meet the stringent battery monitoring requirements of industrial and professional users, what is needed is a method that allows EEPROM-like memories to be used for recording battery status in which the EEPROM data write cycle occurs at a relatively slow rate (once every 10 minutes or longer), yet is assured of having up to the second status data, even in the event of sudden and rapid removal of the battery from the status reporting device.